The present invention relates to a surface acoustooptic wave deflector for deflecting the light which is transmitted in a thin film optical waveguide path due to a surface acoustic wave.
Surface acoustooptic wave deflectors are acoustic optical devices using the technology in which the flux of light which is transmitted in the thin film optical waveguide path is deflected due to the surface acoustic wave generated in this path. Recently, such deflectors have been becoming important more and more as devices for light control.
FIG. 1 is a schematic perspective view showing an example of surface acoustooptic wave deflectors which are used hitherto. A general function of this deflector will now be described hereinbelow with reference to FIG. 1.
In FIG. 1, reference numeral 1 denotes a substrate of an optical waveguide path; 2 is a thin film optical waveguide path; 3 a comb shaped electrode for excitation of a surface acoustic wave; 4a an input photocoupler; and 4b an output photo coupler. A piezoelectric crystal such as, e.g., LiNbO.sub.3 or the like is used as the substrate 1. A layer of a high refractive index formed on the surface of the crystal to have about a few microns is used as the thin film optical waveguide path 2. Or, a non-piezoelectric crystal such as, e.g., Si may be used as the substrate 1. In this case, a thin film formed of a piezoelectric material such as ZnO may be used as the path 2.
A surface acoustic wave 7a excited by the comb shaped electrode 3 is transmitted in the path 2 and crosses a laser beam guided into the path 2 from the input photo coupler 4a, so that the laser beam is Bragg-diffracted due to the periodic change of the refractive index in the optical waveguide path which is caused by the surface acoustic wave.
To excite the surface acoustic wave in such a surface acoustooptic wave deflector, it is necessary to apply a high frequency signal of a desired frequency and a desired electric power to the comb shaped electrode 3. To change a deflection angle in the deflector, it is necessary to cause the frequency of the high frequency signal which is applied to the electrode 3 to be changed within the frequency band of the electrode 3. Hitherto, this kind of high frequency signal has been generated by providing a variable frequency generator 6 and an amplifier 5 in addition to the surface acoustooptic wave deflector.
In FIG. 1, an incident laser beam 8 is transmitted into the path 2 on the substrate 1 due to the input photo coupler 4a (grating coupler in the case of FIG. 1). In the case where the surface acoustic wave is not excited, the incident laser beam is outputted to the outside as a non-diffracted light 9 by the output photo coupler 4b. On the other hand, the high frequency signal generated by the variable frequency generator 6 is amplified to a desired electric power by the amplifier 5 and applied to the electrode 3, by which it is converted to a surface acoustic wave and transmitted in the directions indicated by arrows 11 and 12. The wave surface of the surface acoustic wave 7a transmitted in the direction of the arrow 11 is preset so as to keep the angle between the wave surface and the laser beam such as to satisfy the Bragg condition, thereby causing the Bragg diffraction for the flux of laser beam. The flux of light in the optical waveguide path is deflected due to such an effect and outputted to the outside as a diffracted light 10 by the output photo coupler 4b. The surface acoustic wave 7b after causing the Bragg diffraction for the flux of light and the surface acoustic wave transmitted in the direction of the arrow 12 are reflected by the edge surface of the substrate 1 and absorbed by absorbing materials 13a and 13b to prevent the occurrence of the unnecessary secondary effect. As the oscillating frequency of the variable frequency generator 6 is changed, the wavelength of the surface acoustic wave which is excited by the comb shaped electrode 3, namely, the pitch of the wave surface of the surface acoustic wave 7a varies and the diffraction angle of the flux of light also changes, so that the direction of the output light 10 can be changed.
However, the conventional surface acoustooptic wave deflector as shown in FIG. 1 has the following drawbacks.
1. The variable frequency generator 6 is indispensable. PA1 2. The variable range of frequency of the generator 6 must be wider set than the frequency band of the comb shaped electrode 3 in consideration of the manufacturing deviation and the temperature characteristic, so that it is disadvantageous in terms of the cost and signal purity which is generated, namely, the signal purity of the surface acoustic wave. PA1 3. The surface acoustic wave 12 which is generated and transmitted from the electrode 3 in the direction opposite to the Bragg diffraction area is absorbed by the absorbing material 13b without acting at all; thus, it is wasted. In addition, unless the surface acoustic wave 12 is effectively absorbed, it is reflected by the edge surface of the crystal and transmitted in the same direction as the arrow 11, so that the deflection characteristic is adversely influenced.
It is an object of the present invention to solve at least one of the problems of the conventional technology as mentioned above.
To accomplish this object, a surface acoustooptic wave deflector according to the present invention is constituted in a manner such that in the transmission path of the surface acoustic wave which is excited by the surface acoustic wave exciting means such as the comb shaped electrode for excitation, the surface acoustic wave receiving means such as the receiving comb shaped electrode for receiving the surface acoustic wave is provided, and these exciting means and receiving means and an external circuit including an amplifier form a closed loop and constitute a positive feedback type generator.